This invention pertains generally to CMOS very large scale integrated (VLSI) circuits formed on semiconductor chips and particularly to a CMOS binary difference cell.
The most common method of performing a subtraction of two binary numbers (A-B) is to invert the subtrahend (B) and add it to the minuend (A) in a binary adder, the carry-in input being set to a logic level one. When the binary numbers being processed are N-bit numbers (where "N" is an integer greater than one), N+1 inverter and binary adder stages are required. Each inverter stage occupies space on the semiconductor chip, consumes power and slows operation. Furthermore, the complexity of the design of each binary adder stage is increased to be sure that the carry-in input is at a logic level one when subtraction is being carried out. Finally, it will be appreciated that the N+1 binary adder stage is required for subtraction utilizing two's complement arithmetic on two N-bit numbers to allow sign information of the difference to be provided.